Sen. Ben Sasse, Nebraska Republican, says he can’t support GOP presidential front-runner Donald Trump for president and that he’ll likely look for a third candidate to support if the billionaire businessman ultimately wins the Republican nomination.

“I’m as frustrated and saddened as you are about what’s happening to our country. But I cannot support Donald Trump,” Mr. Sassewrote in an open letter posted to Facebook Sunday.

Mr. Sasse had already traveled to Iowa to urge voters not to support Mr. Trump in the intra-party Feb. 1 caucuses. But he’s now going beyond that, saying he would support neither Mr. Trump nor former Secretary of State Hillary Clinton, the Democratic front-runner, in a hypothetical match-up.

“I sincerely hope we select one of the other GOP candidates, but if Donald Trump ends up as the GOP nominee, conservatives will need to find a third option,” Mr. Sasse wrote.

“Mr. Trump’s relentless focus is on dividing Americans, and on tearing down rather than building back up this glorious nation. Much like President Obama, he displays essentially no understanding of the fact that, in the American system, we have a constitutional system of checks and balances, with three separate but co-equal branches of government,” he wrote.

“And the task of public officials is to be public ‘servants.’ The law is king, and the people are boss. But have you noticed how Mr. Trump uses the word ‘Reign’ — like he thinks he’s running for king? It’s creepy, actually. Nebraskans are not looking for a king. We yearn instead for the recovery of a Constitutional Republic,” Mr. Sasse wrote.

The missive from Mr. Sasse came just hours after Mr. Trump received his first endorsement from a sitting U.S. Senator in GOP Sen. Jeff Sessions of Alabama.

Mr. Sasse also wrote that political parties are not “families” or “religions,” but rather “tools to enact the things that we believe.”

“I was not born Republican. I chose this party, for as long as it is useful,” he wrote.

“If our party is no longer working for the things we believe in — like defending the sanctity of life, stopping ObamaCare, protecting the Second Amendment, etc. — then people of good conscience should stop supporting that party until it is reformed.”